1. Field of the Invention
The present invention generally relates to an optical disk device and particularly relates to an optical disk device for reading and writing information on an optical disk.
2. Description of the Related Art
CD-R (CD-recordable) is an optical disk of a write-once read-many type. A laser beam is irradiated on the optical disk to write information on such a recordable type optical disk. Before writing information on the optical disk, a test is carried out to derive an optimum laser power (OPC: Optimum Power Control) to improve reading accuracy of the written information. The CD-R is provided with a test area divided into 100 partitions to allow 100 times of such tests.
FIG. 7 shows a graph of laser power against time illustrating how a test is carried out for partition (n+1) which is a partition following the already tested partition (n). A reference power value (Pref) prerecorded on the optical disk is read out. Then, centering on the reference power value (Pref), 15 steps of power are determined between 0.7 Pref and 1.3 Pref at equal intervals (ΔP). Test information is recorded on partition (n+1) using the 15 steps of power values. The recorded test information for each power value is read out and the power value indicating the optimum power of the laser diode is determined. Actual information is written on the optical disk using the thus obtained optimum power of the laser diode.
Also, when writing information on the CD-R, the laser diode power is monitored by a photodiode provided adjacent the laser diode so as to control the current supplied to the laser diode such that the laser diode power takes a predetermined value (APC: Automatic Power Control).
Generally, the APC of the optical disk device is operated by driving the laser diode such that a laser beam is not focused on the optical disk at the start of a writing operation. Thus, the amount of current to be supplied, or the quantum efficiency (=light emission power/driving current), is derived. The laser beam is not focused on the optical disk to prevent unwanted information from being written on the optical disk.
If, after having achieved the focus state, the laser diode is still driven with the same electric current as the current obtained in the APC during the non-focused state, the writing power becomes higher than the light emitting power in a non-focused state.
In other words, when a focus servo control is applied to achieve a state where there is a reflected light towards the laser diode, there arises an offset between a setting power and an actual light emitting power.
Thus, according to the invention of Japanese laid-open patent application No. 3-29125, a writing operation is started at a driving current slightly lower than the driving current derived when there is no reflected light, so as to reduce the difference between the setting power and the actual light emitting power. Further, after initiating the writing operation, the actual light emitting power is controlled such that it becomes the same as the setting power.
However, for CD-R, the OPC is carried out with the writing time for each recording level being only 1/75 second at a normal speed (1×). For high-speed recording, the writing time will be further reduced by a factor of the speed. Accordingly, even if-OPC is carried out, there is not enough time to achieve a state where the light emitting power is equal to the setting power, and thus the APC cannot be implemented.
Japanese laid-open patent application No. 11-185513 proposes a method of implementing a constant current drive which aims to obviate such a problem. An APC is operated using a part of the test area to derive the quantum efficiency in a focuses state in advance. Then, using the derived quantum efficiency, the light emitting power of the laser diode during an OPC is made equal to the setting power at an instant the APC is turned off.
When a writing operation is carried out on the optical disk while keeping the power of the laser diode constant, as shown in FIG. 8A, the quantity of the reflected light from the optical disk varies as shown in FIG. 8B.
That is to say, within a range where the recording layer of the optical disk does not fuse, the amount of the reflected light increases in proportion to the laser power.
However, within a range where the recording layer fuses and the information is written on the optical disk, the amount of reflected light decreases as the laser power increases. In other words, as the recording layer starts fusing and the fused area increase, the amount of reflected light decreases until it reaches the amount of reflected light corresponding to the fused area corresponding to the power of the irradiating laser beam.
That is to say, as shown in FIG. 8C, the amount of reflected light against the laser power drastically changes near the optimum recording power since the recording layer starts to fuse. This is shown as a point of inflection in FIG. 8C.
In the invention described in the above-mentioned Japanese laid-open patent application No. 11-185513 also, the amount of reflected light drastically changes for a slight change of the writing power in the region near the optimum recording power. Therefore, even if the quantum efficiency in a focused state is derived in advance and the OPC is carried out at a constant current drive with the APC being turned off, there arises a slight difference between an optimum recording power obtained when a test writing is implemented at a constant current drive and an optimum recording power obtained when a test writing is implemented with the APC being turned on.
Recently, due to a higher recording speed, the margin of the optimum power during a high-speed recording of the optical disk, i.e., the range of power in which an optimum recording is possible, has become narrow. Thus, such a slight difference may cause a serious problem.